" . . tee off the inlet line to the gauge and hook the filter return to a port on the block . . "

If your inlet line is from the rearward port in the block, and your gauge is on a T on the inlet line, your blockage must be in the T or in the line to the T (you reported that the sender line directly connected from the engine-block to gauge gave a reading?).

Disconnect the line from the engine-block to the T, at the T, and crank a little to if oil squirts out that line.

If it does, connect that line to the T, with neither outlet line connected to the T, and crank a little to see if oil squirts out the T.

If it does, connect the sender to the T, and crank a little to see if oil squirts out the T.

If it does, block the port on the T that goes to the filter, crank the engine and see if the gauge reads pressure.

If it does not, that pressure line to the gauge is now blocked.

If there is a pressure reading at the gauge, I am lost - if you now connect a line from the T to the filter, you should still get a reading at the gauge.

Tim
1954Advance-Design.com
1954 3106 Carryall Suburban
1954 3104 5-window pickup w/Hydra-Matic
The 216 and early 235 engines are not "splash oilers" -- this is a splash oiler.